Like most optical sensors, a CCD array responds linearly to light intensity. A charge on the CCD array is directly proportional to the number of light photons striking a particular sensor. However, due to the electronics, the optics, the light source, and the sensor itself, a CCD array with many sensors (5000 for example) does not respond uniformly during data recovery and conversion to electric charge levels. This non-uniformity requires each sensor in the array to be calibrated or normalized to every other sensor through electronic and software compensation. This calibration is accomplished by determining the near saturation level (light output) and the dark level (quiescent output) of each sensor. These two points provide means to mathematically normalize the linear output of all sensors in the array by interpolating between the two values. This is based on an assumption that a linear relationship exists between an electric charge level and light intensity.
Data on a data medium having an optical density (OD) range of 0.0 to 6.0 is typically recovered using sensors which have a dynamic optical density of 3.0 decibels, employing multiple light levels and exposure times. In this case, there has been no automatic means to accurately calibrate the sensors to reference the output digital values to optical densities of the data medium.
Unfortunately, the CCD array sensors do not always exhibit the linear relationship beyond optical density of 3.0 decibels. In fact, major deviations occur near the dark level output. This non-linearity causes deviation in calibration, and consequently makes any reference to optical density inaccurate which may interfere with the data recovery from the data medium, i.e., film.
Furthermore, the CCD array for reading data from the data medium normally possesses a fixed field of view. When reading data from the data medium which is smaller than the fixed field of view of the CCD array, the extraneous illumination beyond the edges of the data medium yields undesirable flare. The flare increases the charge of all sensors in the array reducing the reliability of the process. To reduce the unwanted effect of this flare, manual masking is usually employed around the edges of the data medium.
As stated above, the CCD sensor possesses limited dynamic range of optical density due to the limited number of photons that can be accumulated in each sensor. The number of photons is directly proportional to the intensity of illumination reaching each sensor. Since EQU I=I.sub.0 /10.sup.OD, where
I is the intensity of transmitted illumination, PA1 I.sub.0 is the intensity of incident illumination, and PA1 OD is optical density of the data medium,
the intensity of transmitted illumination is reduced by a factor of 10 for each integral value of optical density, provided that the intensity of the transmitted illumination remains constant.
For example, 99% of the light is absorbed by a data medium of optical density 2.0. Thus, the ability to discriminate one optical density from another, i.e., contrast sensitivity, is extremely reduced after only 2 decades of optical density, leaving only 1% of the light for logarithmic distribution among the remaining optical densities.
To recover data from a data medium possessing greater than 2 decades of density with significant contrast sensitivity, several succeeding scans of the data medium at different light levels and/or different integration periods are necessary. Multiple succeeding scans are time-consuming and cumbersome due to the retrieval and re-insertion of the data medium.
A need, therefore, exists for a digitizing CCD array system which overcomes the above disadvantages of the existing digitizing systems.